Semiconductor-mounting heat-radiative substrates, a method of making and use thereof in semiconductor packages

ABSTRACT

A method for manufacturing heat-radiative substrates on which semiconductor devices such as ICs and transistors are mounted and packages using the substrates, wherein a plurality of CuW or CuMo composite materials obtained by the infiltration method or the mixed powder sintering method are joined together with Cu interposed therebetween. Accordingly, the remaining empty holes within the CuW or CuMo materials are filled sufficiently with Cu, allowing high-quality packages having a successful thermal characteristic to be obtained.

This application is a divisional application of Ser. No. 08/087,073filed Jul. 7, 1993 , now U.S. Pat. No. 5,305,947 which in turn is acontinuation of abandoned application Serial No. 07/770,430, filed Oct.3, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturingsemiconductor-mounting heat-radiative substrates in which a plurality ofCuW or CuMo composite metal alloys having different compositions arejoined together with Cu interposed therebetween and further relates tosemiconductor packages using the substrates.

2. Description of the Prior Art

In the last few years, an increase in operating speed and degree ofintegration of ICs, transistors and the like as well as an increase intheir capacity has involved increasing amounts of heat generated bysemiconductor devices. As a result of this, it matters to a great extenthow generated heat is eliminated and the devices are cooled toaccomplish successful operation thereof. In terms of cooling devices,there is a similar problem also in parts related to semiconductorlasers. In order to solve this problem, composite materials formed ofcombinations of Cu and W, or Cu and Mo have recently come into practicaluse as heat-radiative substrates for mounting devices in packages inwhich semiconductors are accommodated.

These composite materials are manufactured by some methods, for example,(a) in which Cu melted in a reducing atmosphere is infiltrated into aporous product obtained by sintering W or Mo powder (as disclosed inJapanese Patent Laid-Open Publication No. 59-21032), and (b) in which Wor Mo powder is mixed with Cu powder and moreover the result is sinteredin a reducing atmosphere (hereinafter, referred to as mixed powdersintering method).

FIGS. 7 and 8 are sectional views showing typical constructions ofconventional IC ceramic packages in which the above-described compositematerials are used as heat-radiative substrates; and FIGS. 9 and 10 aresectional views showing constructions of such packages for use withtransistors.

In FIGS. 7 and 8, designations are as follows: 21 denotes a substratecomposed of CuW or CuMo; 22 denotes a multi-laminated ceramic substratecomposed of, for example, Al₂ 0₃ layers; 23 denotes an Si semiconductordevice mounted on the substrate 21; 24 denotes a covar (Fe-29%Ni-17%Co)lead wire; 25 denotes a bonding wire; and 26 denotes a package composedof covar or Al₂ 0₃.

The construction shown in FIG. 8 is such that the Si semiconductordevice 23 mounting portion 21a of the substrate 21 is higher than theconstruction in FIG. 7. Accordingly, the terminal mounting portion 27 ofthe multi-laminated ceramic substrate 22 can be flush with the Sisemiconductor device 23, thus advantageous for mounting devices.

The number of laminates of ceramic substrates is recently increasingwith increasing capacity of ICs; the demand for such constructions istherefore increasing more and more.

Conventional transistor-oriented packages are now described, referringto FIGS. 9 and 10. In FIG. 9, on a BeO circuit substrate 28 in thecenter of a CuW or CuMo substrate 21 there is mounted an Sisemiconductor device 23. In FIG. 10, on the other hand, as is the usualcase for use with FETs (Field Effect Transistors) employing GaAstransistors as devices 29, a GaAs transistor device 29 is directlymounted to a protrusive portion 21b provided on a CuW or CuMoheat-radiative substrate 21 having a good heat conductivity and thedevice can be wired with a circuit 31 provided on an insulating ceramicsubstrate 30 in the vicinity of the device so as to be flush with thecircuit, which type of construction is thus preferred.

The substrates 21 used in the packages shown in FIG. 7 and FIG. 9 out ofthe above-described ones are simple flat plates which do not involvemuch difficulty in manufacture thereof, whereas the stepped substratespartly having protrusions 21a and 21b as shown in FIG. 8 and FIG. 10involve various problems on manufacture thereof.

These stepped substrates are manufactured, in general, by leavingprotrusive portions on a sheet of flat plate and removing the remainsthereof by cutting or grinding.

This method, however, is wasteful in material to be cut and, what ismore, burdened with cost for cutting tools due to the fact that itssubstrate material is of combination of Cu, which is a soft material,and W or Mo, which is a hard, difficult-to-cut material, and thereforethe cutting tools are unavoidably subject to intermittent cutting, withthe result of their greatly worn edges, which requires cutting tools tobe frequently exchanged to obtain high precision and suppress anyafter-processing machining distortions.

On the other hand, it is possible to provide these stepped substrates bytaking the above-described methods, that is, by the infiltration method(a) that Cu is infiltrated into a stepped, sintered product of W or Moor by the mixed powder sintering method (b) that mixed powder of W or Moand Cu is molded into a stepped shape and then sintered. However, thesemethods are accompanied by the following problems.

The substrate 21 used for a package having such a configuration as shownin FIG. 8 is shaped as shown in FIG. 11, where the surface 33 brazed tothe ceramic substrate 22 is larger in area than the surface 32 on whichthe semiconductor device 23 is mounted. The surface 33 is often requiredto be as thin as 0.3 to 0.5 mm, normally.

Accordingly, in manufacturing such substrates by cutting off a flatplate so as to leave a protrusive portion thereof, the area of the partto be cut off is large and, when the edge of the cutting tool is worn,the mounting surface 33 is deformed due to machining distortion incutting, thus often encountering an obstacle in brazing it with theceramic substrate 22.

Some other methods may be available to manufacture these substrates. Forexample, one method is that W or Mo powder is molded by die-pressinginto a stepped shape similar to the foregoing one and Cu is infiltratedinto a porous product obtained by sintering the molding result (theinfiltration method). Another is that mixed powder of W or Mo powder andCu powder is molded by die-pressing into a stepped shape similar to theforegoing one and the result is sintered (the mixed powder sinteringmethod). However, as described above, in the shape of FIG. 11, thesurface 33 to be brazed with the ceramic substrate 22 is thin such thata die-pressing result having the same density as thesemiconductor-device mounting surface 32 is difficult to obtain.Moreover, even if a die-pressing result having the same density isobtained, the brazed surface 33, which forms the thin-wall portion ofthe die-pressing result, is low in strength, thus difficult to treat.

In addition, in order to obtain composite materials having no defects(empty holes), high-temperature treatment over the melting temperatureof Cu is necessitated as the baking temperature. In order to fill emptyholes sufficiently with Cu, excessive Cu needs to be added for baking,which causes the baking result to be covered in its surface with Cu.Accordingly, some countermeasure must be taken to remove the resultingCu, requiring labor equivalent to that in manufacturing the substratesfrom flat plates by cutting.

Next, the heat-radiative substrate 21 used for transistor-orientedpackages, having a sectional configuration as shown in FIG. 10, isshaped as shown in FIG. 12, where the protrusive portion 21b on whichthe GaAs transistor device 29 is mounted is much smaller than the flatportion 34.

Therefore, it is difficult to manufacture a die for press-forming such aprotrusive portion and to charge powder into the die for die-pressing,involving some problems in manufacturing.

As a method for manufacturing such a stepped substrate, such one isavailable that the protrusive portion and the flat portion areseparately prepared, subjected to Ni plating (to ensure the wetting ofbrazed material in brazing), and brazed along with the ceramic portionin manufacturing ceramic packages to obtain packages such as shown inFIG. 13. However, in this case, it is difficult to take a constantinterval of a semiconductor-device mounting substrate 35 to a ceramicportion 36, and moreover, due to empty holes remaining within a brazingmaterial 37 which is the joint surface between the substrates 35 and 36and due to the intervening brazing material 37 having a low heatconductivity, heat conduction will adversely be affected. This willcause quality deviations of packages, disadvantageously.

SUMMARY OF THE INVENTION

The inventors made every effort to solve the foregoing problems of theprior arts in obtaining the above-described stepped substrates andpackages with the substrates assembled therein, and thus have achievedthe present invention.

Accordingly, the object of the present invention is to provide a methodfor manufacturing semiconductor-mounting heat-radiative substrates inwhich a plurality of CuW or CuMo composite metal alloys obtained by theinfiltration method or the mixed powder sintering method are joined withCu interposed therebetween and further provide semiconductor packagesusing the resulting substrates.

According to the method of manufacturing a heat-radiative substrate A ofthe present invention, a flat plate 1 serving as the stepped protrusiveportion, as shown in FIG. 1, and a flat plate 2 forming the flat portionare separately prepared.

As a method for preparing them, W or Mo powder is molded, then melted Cuis infiltrated in a reducing atmosphere into empty holes of a porousproduct obtained by sintering the result (the infiltration method), andthe resulting composite metal alloy of W or Mo and Cu is ground into theflat plates 1 and 2. Next, with a thin Cu foil pinched between theseflat plates, they are joined together in a reducing atmosphere while thetemperature is kept higher than the melting temperature of Cu.

In this process, the melted Cu foil melts with the Cu contained in thecomposite metal alloy, excessive part thereof being press-formed out ofthe joint surface. As a result, on the joint surface having been cooled,there is only an extremely thin Cu layer left, thus not affecting theuniformity of thermal expansion all over the joint substrate A.Moreover, since Cu has a good heat conductivity and a melting point(1080°C.) higher than the brazing material used for joining ceramicsubstrates in assembling packages (generally, eutectic brazing material,brazing temperature: 750° to 900° C.), it is possible to providesubstrates which have a thermal characteristic comparable to that groundout from an integral material and which can be assembled by brazing.

Incidentally, the remaining empty holes of the joint surface, which isin question, are free from any defects as in the inside of the metalalloy manufactured by the infiltration method, since the melted Cu foiland Cu contained in the composite metal alloy are integrated together sothat the excessive part thereof covers the entire joint surface, thusexcluding any problems.

Furthermore, by the method in which W or Mo powder is mixed and moldedwith Cu powder, treated at a temperature over the melting temperature ofCu to obtain a composite metal alloy (the mixed powder sinteringmethod), a composite metal alloy having less defects (i.e. empty holes)is not satisfactorily obtained. However, when such a metal alloyresulting from the above method is employed as a material for obtainingheat-radiative substrates of the present invention, Cu melted in joiningwill cover the remaining empty holes within the material, thus theresulting heat-radiative substrate after joint showing less defects(empty holes) enough to serve as the material.

The method of the present invention is advantageous not only in savingthe materials for the composite metal alloys to be used and reducing thecosts for the cutting process and the tools due to reduction incutting-processed margins but also in capability of making a number ofshapes by combinations by means of preparing simple standard dies. Thus,it is suitable for manufacturing such products which are critical tomeet the need for various-type, small-lot production.

The method of the present invention can provide substrates having thefollowing advantages on performance in addition to the above-describedones on manufacture.

According to the joining method of the present invention, it is possibleto vary the composition ratio of materials combined thereby to makesubstrates having such characteristics that cannot be provided by themethod of grinding out them from a single material by cutting.

CuW or CuMo composite metal alloys have the following characteristicsdepending on their composition.

In principle, the higher the content ratio of Cu in a composite metalalloy, the better the heat conductivity and the better the heatradiation characteristic; for use as package substrates, selections ofcomposition ratio as shown below are available depending on the type ofinsulating materials and electrically conductive materials to be used incombination:

As an example, a composite material of Cu: W=10:90 (by weight) has athermal expansion ratio that approximates that of alumina ceramics, thusthe composite material is most suitable for heat-radiative substratesbrazed with alumina ceramics.

Another of Cu: W=15:85 (by weight) has a thermal expansion ratio thatapproximates that of beryllia ceramics, thus used for FET-orientedceramic packages in which Si devices are accommodated.

A CuMo composite metal alloy, although its heat conductivity is inferiorto that of CuW composite materials, is small in specific gravity, thussuitable for use as substrates for large-sized packages.

As seen above, thermal characteristics (heat conductivity and thermalexpansion ratio) differ depending on the composition ratio. Accordingly,if a single material is used, all the characteristics cannot besatisfied as heat-radiative substrates for use in packages.

By contrast, according to the present invention, a plurality of requiredcharacteristics can be satisfied as follows.

In general, used as substrates for use in Si FET-oriented ceramicpackages, where good thermal and insulating characteristics arerequired, are ceramic packages of such a construction that Al₂ O₃ isused as the multi-layer insulating ceramic substrate 22 of outer wallsand BeO is used as the heat-radiative insulating circuit substrate 28 onwhich the semiconductor device 23 is mounted, as shown in FIG. 9.

In this case, the Al₂ O₃ and the BeO circuit substrate 28 differing fromeach other in thermal expansion ratio as outer walls are necessarilysimultaneously joined with the CuW substrate 21 by brazing. Accordingly,in order to prevent the two materials 22 and 28 from damage due tothermal distortion in cooling subsequent to brazing and prevent the CuWsubstrate 21 from deformation, the composition ratio is fine adjusteddepending on design configuration such as Cu: W=13:87 or Cu: W=15:85;however, this cannot serve as satisfactory settlements for problemsinvolved.

On the other hand, according to the method of the present invention, itis possible to make CuW composite metal alloys formed of combinations ofmaterials having different ratios of Cu to W, such as Cu: W=20:80 forthe flat plate 1 and Cu: W=10:90 for the flat plate 2, as shown inFIG. 1. As a result, even though two types of materials of Al₂ O₃ andBeO are brazed at the same time, problems of damage and deformationthereof due to thermal distortion can be solved.

It is to be noted here that the combinational substrates of differentcomposition ratios made by the method of the present invention have asoft Cu thin layer existing between the two materials, and this thinlayer serves as a buffer layer so that almost no deformation will occurdue to the difference in thermal expansion ratios of the two materials.

In a heat-radiative package as shown in FIG. 2, when a ceramic circuitsubstrate 3 of Al₂ O₃ or the like is soldered with the flat plate 2within the package, it is preferable to use a CuW base metal from theviewpoint of thermal expansion, but this will add weight thereto andtherefore CuMo base metals have conventionally been used. This being thecase, according to the present invention, it is allowed to combine CuMomaterial as the flat plate 2 and CuW material as the flat plate 1 with aCu joint 4, as shown in FIG. 3, thus permitting the making of packageswhich are lightweight and suitable for joining with the ceramic circuitsubstrate 3 of Al₂ O₃ or BeO.

In addition, in FIGS. 2 and 3, reference numeral 5 denotes an outerframe made of covar or the like, 6 denotes a covar lead wire, and 7denotes a glass seal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome apparent from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings, in which:

FIG. 1 and FIG. 6 are explanatory views showing the constructions ofheat-radiative substrates according to the methods of the presentinvention;

FIGS. 2 through 5 are sectional views of semiconductor packages usingheat-radiative substrates of the present invention;

FIGS. 7 through 10 and FIG. 13 are sectional views of semiconductorpackages using conventional heat-radiative substrates;

FIGS. 11 and 12 are explanatory views showing the constructions ofconventional heat-radiative substrates; and

FIGS. 14 and 15 are microphotographs showing the structure of the jointsurface section in the heat-radiative substrate of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is now described below in detail by way ofpreferred embodiments thereof.

Embodiment 1

Mixed powder in which camphor was mixed as an organic binder at a mixingratio of 2% by weight with W powder was filled to a die, which was thenpressed to obtain a die-pressed product. This die-pressed product washeated at 500° C. in a hydrogen atmosphere to evaporate the camphoraway, and sintered at 1400° for 2 hours. Thus, a W porous product wasobtained.

Subsequently, a Cu plate having enough weight to fill the holes of theresulting porous product was overlaid thereon and heated at 1200° C. ina hydrogen atmosphere, and Cu was melted and infiltrated into the holesof the porous product. Thus, W and Cu composite metal alloys wereobtained which had a ratio of Cu: W=10:90 (by weight) with their 10mm×10 mm×1 mm and 30 mm×30 mm×0.5 mm peripheral surfaces covered withCu.

Both surfaces of these composite metal alloys were ground and theirperipheral end surfaces were cut to remove Cu. Thus, 9 mm×9 mm×0.8 mmand 29 mm×29 mm×0.3 mm flat plates 1 and 2 were obtained.

Between the resulting two flat plates 1 and 2, a 9 mm×9 mm×0.1 mm Cuplate 4 was set within a carbon jig, and heated at 1200° C. for 10minutes in a hydrogen atmosphere to melt Cu. Thereafter, it was cooled.Thus, a stepped heat-radiative substrates as shown in FIG. 6 wasobtained. After the Cu adhering to the periphery of joint portion wascut away, it was ground by barrel finishing to eliminate the Cu adheringto the periphery.

Looking into the microstructure of the joint portion section of theresulting substrate, it was found as shown in FIG. 14 that such emptyholes were not present in the joint portion as would adversely affectheat radiation.

This surface was plated with Ni, and the result was joined by brazingusing silver solder with an Al₂ O₃ substrate 8 the joint surface ofwhich is W metallized and Ni plated. Thus, an IC ceramic package asshown in FIG. 4 was obtained.

Embodiment 2

A flat plate 1 having a shape of 7 mm×7 mm×0.5 mm and a composition ofCu: W=20:80 and a flat plate 2 having a shape of 30 mm×11 mm×1 mm and acomposition of Cu: W=10:90 were prepared by the same method as inembodiment 1.

Using a stepped substrate in which a 7 mm×7 mm ×0.1 mm Cu foil 4 waspinched between the above two flat plates 1 and 2 and joined by the samemethod as in embodiment 1, an alumina frame 8 and a beryllia plate 9were brazed thereto at the same time, thus making an FET-orientedpackage as shown in FIG. 5.

The resulting package proved to have no cracking in the beryllia plateand moreover the warp of the bottom surface of the package was less than0.002 mm. Thus, a package having a successful heat radiationcharacteristic was obtained.

Embodiment 3

Mixed powder in which camphor was mixed as an organic binder at a mixingratio of 2% by weight with Mo powder was filled to a die, which was thenpressed to obtain a die-pressed product.

This die-pressed product was heated at 500° C. in a hydrogen atmosphereto remove the camphor, and then sintered at 1200° C. for 2 hours in ahydrogen atmosphere. Thus, an Mo porous product was obtained.

Subsequently, Cu was infiltrated into the holes of the Mo porous productin the same manner as in embodiment b 1, so that a Cu and Mo compositemetal alloy having a shape of 30 mm×60 mm×1 mm and a ratio of Cu:Mo=15:85 (by weight) was obtained. By grinding this metal alloy as inembodiment 1, a 29 mm×59 mm×0.8 mm flat plate 2 was obtained.

Further, in addition to this, a flat plate 1 of Cu: W=10:90 and 20 mm×20mm×0.5 mm was obtained by the same manner as in embodiment 1.

With a 20 mm×20 mm×0.1 mm Cu foil pinched between the two flat plates 1and 2, the plates were joined together by the same manner as inembodiment 1.

Since the flat plates 1 and 2 were joined together with a defect-freethin Cu layer 4 interposed therebetween as shown in FIG. 6, aheat-radiative substrate was obtained which was suitable for use with apackage as shown in FIG. 5.

Looking into the microstructure of the joint portion section of theresulting substrate with a microscope, it was found that there were noempty holes in the joint portion such as to adversely affect heatradiation, as shown in FIG. 15.

As described above, according to the methods of the present invention, aplurality of CuW or CuMo materials having different compositions andmade by the infiltration method or the mixed powder sintering method arejoined together with a Cu plate or foil interposed therebetween, and asa result thereof, the remaining empty holes within the CuW or CuMomaterial are filled with melted Cu, thus allowing heat-radiativesubstrates excellent in performance to be obtained.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to be notedhere that various changes and modifications will be apparent to thoseskilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention as definedby the appended claims, they should be construed as included therein.

What is claimed is:
 1. A semiconductor-mounting heat-radiative substratewhich is a joined body of a plurality of CuW or CuMo composite metalalloy portions with a pore-free Cu thin layer interposed therebetween.2. A semiconductor-mounting heat-radiative substrate according to claim1, wherein a plurality of CuW or CuMo composite metal alloys havingdifferent compositions are used as said CuW or CuMo composite metalalloys.
 3. A semiconductor-mounting heat-radiative substrate which is ajoined body of a plurality of CuW or CuMo composite metal alloy portionswith a pore-free Cu thin layer interposed therebetween which is formedby joining together a plurality of CuW or CuMo composite metal alloysobtained by an infiltration method or a mixed powder sintering method,wherein Cu is disposed on an interface between said CuW or CuMocomposite metal alloys and said plurality of CuW or CuMo composite metalalloys are joined together through Cu by heating them to the meltingpoint of Cu or higher in a reducing atmosphere.
 4. A semiconductorpackage comprising the heat-radiative substrate of claim 1 and enclosuremembers mounted on the substrate.
 5. A semiconductor package comprisingthe heat-radiative substrate of claim 2 and enclosure members mounted onthe substrate.
 6. A semiconductor package according to claim 4, whereinthe thermal expansion coefficient of the metal alloy portion of theheat-radiative substrate approximates the thermal expansion coefficientof the adjacent enclosure member.
 7. A semiconductor package comprisingthe heat-radiative substrate of claim 2 herein the package comprises ajoined body of a plurality of CuW and CuMo composite metal alloyportions with a pore-free Cu thin layer interposed therebetween andenclosure members mounted on the substrate.